Fibrous transition metal phosphates

ABSTRACT

THIS INVENTION ENCOMPASSES FIBROUS, COLLOIDAL, CRYSTALLINE, WATER-INSOLUBLE PHOSPHATES HAVING AT LEAST TWO TRANSITION METAL IONS IN THEIR +4 VALENCE STATE WHERE 20 TO 98% OF THE METAL ION CONTENT ON AN ATOMIC BASIS IS A QUADRIVALENT ION OF TITANIUM, ZIRCONIUM, HAFNIUM, MOLYBDENUM, TUNGSTEN, TIN, LEAD, THORIUM OR URANIUM AND THE REMAINING PERCENTAGE BEING +4 VALENT CERIUM IONS WITH THE RATIO OF PHOSPHATE GROUPS TO TOTAL METAL ATOMS BEING FROM 2:1 TO 1.5:1.

United States Patent O 3,666,514 FIBROUS TRANSITION METAL PHOSPHATES Paul Clifford Yates, Wilmington, Del., assignor to E. I. du Pont de N emours and Company, Wilmington, Del. No Drawing. Filed Apr. 1, 1969, Ser. No. 812,425 Int. Cl. C09c 1/00, 1/14, 1/31 U.S. Cl. 106-288 B 8 Claims ABSTRACT OF THE DISCLOSURE This invention encompasses fibrous, colloidal, crystalline, water-insoluble phosphates having at least two transition metal ions in their +4 valence state where 20 to 98% of the metal ion content on an atomic basis is a quadrivalent ion of titanium, zirconium, hafnium, molybdenum, tungsten, tin, lead, thorium or uranium and the remaining percentage being +4 valent cerium ions with the ratio of phosphate groups to total metal atoms being from 2:1 to 1.521.

BACKGROUND OF THE INVENTION Phosphate inorganic ion exchangers of the prior art have generally been amorphous materials characterized by relatively high ion exchange capacities, but limited because of their poor hydrolytic stability, which is characteristic of the amorphous state. Crystalline ion exchangers, such as microcrystalline plates of zirconium phosphate or the fibrous cerium phosphates of the prior art have substantially improved hydrolytic stability, but because of the closeness of packing of the crystal lattice have had limited exchangeability because of steric considerations. For example, large organic and inorganic ions have a 'very limited access into the interior portions of the structure of these ion exchange materials. Further, because of the very high regularity of packing of the alternate phosphate and metal layers in such a structure, the electrical resistivity of such compositions have been quite high and diifusion rates of ions through such structures have been undesirably long.

The compositions of this invention have an excellent balance between the high exchange capacity and ability to perform ion exchange on a variety of ions of different sizes, which is characteristic normally only of the amorphous compositions of the prior art, with the high hydrolytic stability and mechanical strength which has heretofore been characteristic only of the highly crystalline compositions of the prior art. This is thought to be due to the fact that by introducing a controlled irregularity in packing as a result of employing ions of two difierent sizes in the structure, sufiicient crystalline characteristics can be obtained to stabilize the structure, but sufficient disorder can be introduced to avoid the low electrical conductivity and the selective exclusion of large ions hitherto characteristic of crystalline ion exchangers of this class.

In addition, the great flexibility which can be introduced as a function of substituting ions of slightly different ion sizes, and substituting 2, 3, or even more quadrivalent transition metal ions of different characteristics into the lattice structure of the compositions of the invention, permits a great deal of freedom in tailoring an ion exchange material to specific separations not hitherto possible. By a suitable choice of both the quantity and the nature of the substituting ion in the structure, a Wide variety of highly specific ion exchange materials can be produced.

It is also generally true that substitution of these other quadrivalent metal ions into the structure of the compositions of the invention results in stronger and higher modulus compositions than the compositions of the art. This 3,666,514 Patented May 30, 1972 DESCRIPTION OF THE INVENTION The compositions of this invention are fibrous, colloidal, crystalline, water-insoluble phosphates. They are characterized by having two or more transition metal ions in the +4 valence state in their structure, with from 20 to 98% of the metal ion content on an atomic basis being one or more ions selected from the group of the quadrivalent ions of titanium, zirconium, hafnium, molybdenum, tungsten, tin, lead, thorium, and uranium, and from to 2 atom percent of the metal ions in the structure being +4 valent cerium atoms.

The ratio of phosphate groups to total metal atoms in the structure can range between 2 to 1 and 1.5 to l, with compositions containing from 1.8 to 1 to 2 to 1 being preferred. The compositions of the invention will normally also contain the equivalent of approximately two molecules of water per metal atom and may, in addition, contain further amounts of water in an adsorbed state.

The compositions of the invention are crystalline, negatively charged, colloidal fibers. They are characterized by a very high length to diameter ratio and, in general, will have lengths at least ten times and up to many thousand times greater than their diameters.

The diameters of these particles will generally range between 5 and 100 millimicrons, and it is preferred that they be in the range of from 10 to 50 millimicrons. Fiber lengths in excess of a micron are also preferred. It will be noted that the average fiber dimensions described above refer to the dimensions of the individual fibrils, but that the' products of the invention can also be aggregated into fiber bundles with long axes arranged parallel, which can have diameters substantially in excess of the individual fiber diameter. The surface area of the products of the invention depends somewhat on the degree of aggregation, but will generally range between 10 and 100 m. /g., with products having surface areas between 20 and 80 mF/g. being preferred.

The products of the invention when suspended in aqueous solutions are characterized by a negative electrostatic charge, as shown by their ability to coagulate positively charged colloidal materials and to adsorb positively charged inorganic or organic ions. The products of the invention have a substantial ion exchange capacity which depends to some degree on the degree of aggregation and the time of contact allowed for the ion exchange process, and is also influenced by the pH at which the ion exchange operation is cond-ucted. In general, the ion exchange capacity will range from one milliequivalent per gram to as high as about 7 milliequivalents per gram. However, under identical conditions, the products of this invention will give better ion exchange capacity than pure cerium phosphate fibers.

Broadly, the processes of preparing these novel compositions will involve mixing a solution of a soluble salt of quadrivalent cerium with a soluble salt of at least one of the +4 valent transition metal ions previously noted, and contacting this mixture of quadrivalent ions with concentrated phosphoric acid. Suitable soluble salts include the sulfates, the nitrates, the halides, and, in general, any water-soluble salt which forms stable aqueous solutions of the respective ions.

The rate of formation of the products of the invention will be proportional to the concentration of the salts employed, the concentration of phosphoric acid employed, and. the temperature. When the quadrivalent ion salt solutions are mixed with phosphoric acid solutions at relatively low temperatures an intermediate gel phase will be formed which may consist of exceedingly fine diameter fibers or it may consist of amorphous water-insoluble gels. In any event, such materials can be grown to substantial size, and their crystalline character increased by either simultaneous or subsequent heating.

A preferred procedure for preparing the compositions of the invention is to mix dilute mixtures of the quadrivalent metal salts into a heated, stirred solution of concentrated phosphoric acid. Six normal phosphoric acid solutions are suitable, although concentrations ranging from as little as 0.5 normal to normal can be employed.

The temperature of mixing can vary from 0 C. to several hundred degrees centigrade, with temperatures of from about 80 to 130 C. being preferred. Exceedingly long reaction times, coupled with very high temperatures are not preferred, because of the risk of disproportionation of the products of the invention into the individual phosphates rather than the mixed phosphates which are desired. If the compositions of the invention are heated in an autoclave for prolonged periods of time it is possible, for example, for a cerium-zirconium phosphate of the invention to disproportionate into a phase which is rich in cerium phosphate and a second phase which is rich in zirconium phosphate, rather than the homogeneous fibrous crystal which is desired. Thus when temperatures above the boiling point of water are employed, it is desirable to minimize the time of exposure to such temperatures. Suitable times are from a few minutes to about an hour, at temperatures about 250 C. with proportionately longer times being possible as the temperature of the reaction is allowed to drop to, for example, 100 C.

In all instances, it is desirable to maintain a substantial excess of phosphoric acid relative to the concentration of quadrivalent metal ions employed, and this can be conveniently done by delivering the solution of quadrivalent metal ions into the previously prepared phosphoric acid solution. The amount of phosphoric acid in the solution should be at least 50% greater than that stoichiometrically required to react with the metal ions, assuming the formula of the product would be 2 moles of phosphate for each mole of transition metal ion. As a practical matter larger excesses up to even a hundred times that required by this criterion are not objectionable but extreme excesses larger than this are undesirable. This is true particularly when operating in concentrated solutions, since too large an excess of phosphoric acid will start to dissolve the cerium phosphate or other transition metal phosphates in the form of complex soluble metal phosphates containing more than two phosphate atoms per metal atom.

The compositions of this invention are useful as reinforcing fillers for a veriety of organic plastics in which their fibrous form, their colloidal character, and their ion exchange capacity furnish a unique combination of properties. By taking advantage of their ion exchange capacity and substituting positively charged ions, either inorganic or organic, into the structure, these ions in turn being capable of interaction with the organic resins to form a permanently stable chemical bond, it is possible to produce reinforcing fillers which have inherent coupling agents, or groups capable of coupling to the resins, built in on their surfaces. For example, it is possible to ion exchange at least the surface of the compositions of the invention with a quaternary ammonium organic salt containing unsaturated linkages in the hydrocarbon portion of their structure. These organic groups are bonded by stable, salt-like bonds to the negatively charged fibrous transition metal phosphates of the invention, and the unsaturated hydrocarbon linkages in their structure can, in turn, be copolymerized with resins such as acrylic resins, polyester resins, and the like, as well as with resins containing vinyl unsaturation by mechanisms known in the art for inducing the copolymerization of organic unsaturated compounds with one another.

Alternatively, inorganic ions such as cationic complexes of zirconium with methacrylic acid or chromium with methacrylic acid and the like can be permanently bonded to the surfaces via ion exchange and the unsaturated linkages in the methacrylic acid in turn can react with the unsaturated groups in the resins.

The very high length to diameter ratio of the compositions of the invention give them a high efficiency as reinforcing fillers, while their extremely small particle size facilitates ease of fabrication of filled resin by techniques such as injection molding. The phosphate content of the compositions of the invention imparts desirable fire-resistant qualities to the resulting filled resin composition.

The compositions of the invention are useful to prepare refractory inorganic ion exchange papers, thread, cloth, and molded objects. These materials are not easily decomposed by heat, and possess high melting points. Their very high length to diameter ratios and their colloidal character makes-it possible to prepare threads, cloth, and paper-like products in a convenient fashion.

The compositions of this invention are particularly useful as ion exchange resins, combining a high ion exchange capacity with very rapid ion exchange behavior, because such a large fraction of the exchangeable groups is located on the surface. The fibrous character of the compositions of the invention facilitates the preparation of ion exchange membranes, cloth, filter papers and the like, which are well suited for'many ion exchange separations because of their convenient physical form.

EXAMPLE 1 Twenty-six grams of zirconium carbonate and 21 grams of cerium sulfate are dissolved in 2.5 liters of 0.5 molar sulfuric acid. This is added dropwise to 2.5 liters of 6 molar phosphoric acid, which is vigorously stirred with a paddle stirrer and maintained at 94:4" C. The mixed zirconium and cerium sulfate solutions are delivered at an average rate of about 325 cc. per hour, and following the addition, the product is heated to about C. for an additional 4 to '5 hours. It is cooled overnight, and the resulting fibrous particles are filtered and washed several times with distilled water. It is finally air-dried, and the yield is 32 grams of a fibrous, rigid, paper-like material.

The percentage of phosphorus in the product is 17.60%, which is the theoretical value for a composition containing 60 mole percent cerium and 40 mole percent zirconium in a mixed zirconium-cerium phosphate, having a ratio of 2 phosphate atoms per each metal atom and two waters of hydration per metal atom. The above formula is also confirmed by an analysis for the zirconium and for the cerium content of the composition.

Electron micrographs show long fibrous crystals having diameters between 5 and 40 millimicrons and lengths from 10 to 1000 times longer than the width.

This material is an excellent ion exchange membrane, showing a higher capacity for ion exchange, a higher electrical conductivity, and greater strength and modulus than a corresponding composition consisting solely of cerium phosphate. Its hydrolytic stability is improved over corresponding amorphous phosphate materials of similar composition chemically, e.g. over amorphous cerium phosphate. The stiffness and strength of this material is also greater than that of a sheet prepared from either amorphous or fibrous cerium phosphate.

EXAMPLE 2 Three solutions are prepared. The first solution contains 8.3 grams of cerium sulfate dissolved in 500 mls. of 0.5

molar sulfuric acid. The second solution contains 41.8 grams of zirconium carbonate dissolved in 2000 ml. of 0.5 molar sulfuric acid, whereas the third solution contains 2.5 liters of 6 molar phosphoric acid. This last solution, the phosphoric acid, is heated to 94 C. and the solution of ceric sulfate is delivered over a one-hour period into the heated phosphoric acid. Then addition of the solution containing the zirconium sulfate is started, an after 25 minutes, or after 110 cc. of the zirconium sulfate has been delivered, a pronounced schlieren efl'ect is observed in the product. This solution is fed in continuously over the next hours and is heated an additional 4 hours after the zirconium-containing solution is in. The solution is cooled overnight and filtered and washed by reslurrying. It is air dried, and 26 grams are recovered. Electron micrographs show the resulting products to contain fibrous particles which appear somewhat more ribbon-like and somewhat more aggregated than the product of Example 1. It is interesting that even though the mole ratio of zirconium to cerium in this product is 40 to 1, the product is fibrous in character rather than being amorphous and spherical, as would be a similar product containing only zirconium.

EXAMPLE 3 Two hundred and thirty-seven grams of anhydrous titanium chloride are added to 2.5 liters of a 1 molar sulfuric acid solution which also contains 0.125 mole of ceric sulfate. This is delivered dropwise into 2.5 liters of 6 molar phosphoric acid heated at a temperature of 94 C. over a period of 5 hours. Electron micrographs show the product to consist of elongated flakes or ribbons or thin plate-like fibers. Even though the ratio of titanium to cerium in this composition is approximately 40 to 1, this product is totally different in appearance than the amorphous titanium phosphates which are obtained under identical conditions in the absence of the cerium.

EXAMPLE 4 A solution is prepared containing 0.05 mole per liter of ceric sulfate and 0.05 mole per liter of stannic ions derived from anhydrous stannic chloride in half molar sulfuric acid. Five liters of this are added dropwise to 5 liters of 6 molar phosphoric acid while the temperature of the solution is maintained at 94 C. Addition is completed over an eight-hour period, followed by an additional four hours of stirring. The resulting fibrous product contains stannic ions and cerium ions in approximately equal proportions, with the overall composition conforming closely to a half mole of cerium and a half mole of tin for every two moles of phosphoric acid and one mole of water. Electron micrographs show the products to be crystalline fibers, somewhat larger and flatter in appearance than cerium phosphate fibers. Studies of the ion exchange capacity of this material show that it has a considerably higher capacity than an equal weight of cerium phosphate fibers. When prepared in the form of an ion exchange membrane, this composition shows better electrical conductivity by several orders of magnitude than does an identical membrane prepared from fibrous cerium phosphate.

What is claimed is:

1. A negatively charged homogenous fibrous crystal of a metal p'ohsphate having from 20-98 atom percent of one or more transition metal ions in their +4 valence state, selected from the group consisting of titanium, zirconium, hafnium, molybdenum, tungsten, tin, lead, thorium and uranium with the remaining metal ions being +4 valent cerium ions, and the ratio of phosphate groups to total metal ions being between 2 to l and 1.5 to l.

2. A crystal as in claim 1 where the ratio of phosphate groups to total metal atoms is from 2 to 1 to 1.8 to 1.

3. A crystal as in claim 1 where said fibrous crystals have a diameter of 5 to millimicrons.

4. A crystal as in claim 3 where the length of said fibers is at least 10 times its diameter.

5. A crystal as in claim 1 where said metal ions are present on a percent atomic basis of 20 to 98 percent of zirconium and the remaining metal ions being said cerium atoms.

6. A crystal as in claim 1 where said metal ions are present on a percent atomic basis of 20 to 98 percent of titanium and the remaining metal ions being said cerium atoms.

7. A crystal as in claim 1 where said metal ions are present on a percent atomic basis of 20 to 98 percent of tin and the remaining metal ions being said cerium atoms.

8. A process for preparing homogenous fibrous metal phosphate crystals which comprises mixing aqueous solu tions of a salt of +4 valent cerium and at least one salt of a metal in its +4 valence state selected from the group consisting of titanium, zirconium, hafnium, molybdenum, tungsten, tin, lead, thorium or uranium with a stoichiometric excess of phosphoric acid and forming said fibrous crystals by increasing the temperature of said mixture to a temperature no greater than that which will disproportionate the fibrous crystals into individual metal phosphates, said metal salts and cerium salts are present in an amount to give 20 to 98 percent on an atomic basis of the +4 valent metal and the remaining amount being cerium atoms and said excess phosphoric acid being present in a quantity to yield a homogenous fibrous metal phosphate having a ratio of phosphate groups to total metal atoms of between 2 to 1 and 1.5 to 1.

References Cited UNITED STATES PATENTS 9/1948 Bond et a1 252462 JAMES E. POER, Primary Examiner US. (2.1. X.R. 

